July 20, 2011
09:00 AM (EDT)

News Release Number: STScI-2011-23

NASA's Hubble Discovers Another Moon Around Pluto

July 20, 2011: These two images, taken about a week apart by NASA's Hubble Space
Telescope, show four moons orbiting the distant, icy dwarf planet Pluto. The
green circle in both snapshots marks the newly discovered moon, temporarily
dubbed P4, found by Hubble in June. P4 is the smallest moon yet found around
Pluto, with an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison,
Pluto's largest moon Charon is 746 miles (1,200 km) across. Nix and Hydra are
20 to 70 miles (32 to 113 km) wide. The new moon lies between the orbits of Nix
and Hydra, two satellites discovered by Hubble in 2005. P4 completes an orbit
around Pluto roughly every 31 days.

The new moon was first seen in a photo taken with Hubble's Wide Field
Camera 3 on June 28, 2011. The sighting was confirmed in follow-up Hubble
observations taken July 3 and July 18. P4, Nix, and Hydra are so small and so
faint that scientists combined short and long exposures to create this image of
Pluto and its entire moon system. The speckled background is camera "noise"
produced during the long exposures. The linear features are imaging artifacts.
The Hubble observations will help NASA's New Horizons mission, scheduled to
fly through the Pluto system in 2015. Space Telescope Science Institute director's discretionary time was allocated to make the Hubble observations.